I had to go out to my shop and root around in my flight box looking for my reamers, I found three of them. One is a smaller one for my small glow engines. It's a three step reamer. Then I have another one that is a four step reamer, as I recall it is SAE. Then I found a bigger reamer, a three step that goes up to 10MM. I either bought all three of them at my LHS or Tower?? I have had them for a number of years so the old memory is a bit off. Off the top of my head an M12 drill bit is 10.2MM. Knowing what I have been going through I wouldn't trust anything off the top of my head.